1. Field of the Invention
The present invention relates to a liquid crystal display device using no polarizing plate, and a method for driving it. More specifically, it relates to a light scattering type liquid crystal display device which can control a voltage to be applied to a liquid crystal, whereby a texture state of the liquid crystal is changed to bring about light scattering and to thereby control a transmittance of the light, and a method for driving the liquid crystal display device. This display device has a wide visual angle owing to the light scattering type and also enables a reflective type display.
2. Description of the Related Art
At present, in a liquid crystal display device using an electro-optic effect, there is usually employed a TN cell having a twist nematic structure or an STN (supertwist nematic) cell. The display systems of these cells are disclosed in M. Shadt and W. Helfrich, "Appl. Phys. Lett.", Vol. 18 (1971) and T. J. Scheffer and J. Nehring, "Appl. Phys. Lett.", Vol. 45, p. 1021 (1984). As a method for driving these liquid crystal display devices, there can be utilized a simple matrix driving method using a voltage averaging technique, or a driving method using switches of TFTs (thin film transistors) formed at every pixel.
With regard to the simple matrix drive, there have been suggested bistable nematic type liquid crystal display devices having a twist angle of 180.degree. in Japanese Patent Publication No. 51818/1989 (U.S. Pat. No. 4,239,345) and Japanese Patent Application Laid-open No. 230751/1994.
With regard to the liquid crystal display device using no polarizing plate, a DSM (dynamic scattering mode), a PDLC (polymer dispersing liquid crystal mode), a PC (cholestric-nematic phase change mode) and the like have been suggested. The DSM has been invented in 1968 by G. H. Heilmeier et al.
According to the DSM, an ionic current is allowed to flow through a liquid crystal composition to deform the alignment of a liquid crystal molecule, thereby scattering a light. As features of this display method, a liquid crystal material having a negative dielectric anisotropy is used. This method has some problems. For example, a driving voltage is high; a certain current is required; a driver for driving the liquid crystal display device consumes a large electric power; and a conductive material is added to the liquid crystal material, so that the deterioration of the crystal liquid material easily occurs.
In the PDLC, a liquid crystal compound is used in the state of microcapsules. This PDLC has been filed for a patent in 1981 by Fargason et al., and afterward, various dispersion systems of polymeric materials and the liquid crystal materials have been researched.
With regard to the PDLC, the driving voltage is high, as in the DSM, and particularly there is a problem that a saturated voltage is high. As to a response time, a fall velocity is high but a rise velocity is slightly low, and there is a problem that, in voltage-transmittance properties, hysteresis exists.
The PC has been investigated in 1973 by T. Ohtsuka et al. and in 1974 by W. Greubel, and a technique for largely stabilizing the hysteresis has been suggested in 1985 by Mochizuki et al.
The PC (phase change mode) can increase the hysteresis in the voltage-transmittance properties increases, can possess a memory ability and enables the driving under bistability and the display of a large capacity. However, this method has a problem that a gradation display cannot be given owing to the bistability.
The TN mode and the STN mode which have mainly been used as the present liquid crystal display systems have the problems of the narrow visual angle and the low light transmittance. These problems are inherent in the display mode using two polarizing plates. In order to solve these problems, there is a technique of raising the brightness of a back light installed on the back surface of the liquid crystal display device. In this method, however, the electric power to be consumed increases. In consequence, the feature of the liquid crystal display that the consumption of the electric power is small is lost sometimes.
On the other hand, as one of the liquid crystal display devices using the polarizing plates, there have been suggested bistable nematic type liquid crystal display devices in Japanese Patent Publication No. 51818/1989 and Japanese Patent Application Laid-open No. 230751/1994. In the former, the principle of switching of the device is only described, and the response time of the display device is extremely long. The latter describes the driving method of a simple matrix in detail, and also discloses the display device in which the response time is extremely shortened. In each of the above techniques, the two polarizing plates are used, and the display is accomplished by the utilization of electro-optic characteristics on the basis of a birefringence effect. These techniques can more largely expand the visual angle as compared with the STN system, but owing to the bistability, the gradation display is difficult. Furthermore, since the polarizing plates are used, a light utilization efficiency is poor. In order to solve these problems, a technique of raising the brightness of the back light can be employed, but in this case, the consumption of the electric power increases, and the feature of the small electric power consumption in the liquid crystal display device is lost.